Field of the Invention
The invention relates to a crystalline structure with three-dimensional photonic band gap. It also relates to a manufacturing method for said structure.
Recent discoveries have allowed emphasizing a particular physical phenomenon, which develops within periodic dielectric structures, such as some crystals., wherein differences of electrical properties are periodically repeated through their thickness. This phenomenon has been called xe2x80x9cphotonic band gapxe2x80x9d, or xe2x80x9cPBGxe2x80x9d (by analogy with the electronic band gap which can be created in semiconductor crystals). Structures with a photonic band of this type will hereinafter be referred to as xe2x80x9cphotonic crystalsxe2x80x9d.
Exemplary, non-exhaustive structures of this type were described in the following article or book;
the book xe2x80x9cPhotonic Crystals, Molding the Flow of Lightxe2x80x9d, J. D. JOANNOPOULOS and al. xe2x80x9cPrinceton University Pressxe2x80x9d, 1995; and
the article: xe2x80x9cPhotonic Band gap materialsxe2x80x9d, in xe2x80x9cProceedings of the NATO Advanced Institute-on Photonic Band gap Materialsxe2x80x9d, ELOUNDA, Greece, June 1995.
Although one-dimension photonic crystals have been used for long, the idea of creating two- or three-dimension photonic band gap structures only appeared some ten years ago. Ever since, and due to the considerable possibilities, they offer, the interest for photonic crystals has been growing. Indeed, photonic crystals are appropriate for a large number of applications and they are actually in use in devices such as semiconductor lasers, solar cells, high quality resonant cavities, and filters.
Due to their remarkable features, the photonic crystals have found an application more recently in hyper frequency circuits or in components operating in the millimeter or sub-millimeter wave ranges. More particularly, such crystals have found an advantageous application in the so-called photonic band gap antennas, where they are used as substrates for antenna systems.
In an antenna of this type, the underlying photonic crystal prevents that electromagnetic waves of the considered frequency that reach it to propagate therein, and thus it reflects towards the surrounding air the entire electromagnetic radiation emanating from the system it supports, inasmuch as the antenna working frequency matches the band gap frequency.
It is thus effectively possible to consider the photonic crystals as being the electromagnetic homologues of the semiconductor crystals, so far as their behavior towards the electrons is concerned. The photonic crystals have a frequency band for which the propagation of electromagnetic wave through them is inhibited. The operating parameters of the photonic band gap structures are to be found in the periodicity of the variation of their dielectric properties, in the dielectric contrast and in the achieved structural arrangement.
The photonic crystals offer very substantial opportunities for the future, since they allow reducing the global mass of the system where they are used and improve the radiation efficiency performances that are essential for systems operating in a radio frequency or optical frequency range of a predetermined extent.
Those photonic band gap crystals the structure of which is only two-dimensional offer the substantial advantage of being easy to produce. But, although they can be useful sometimes, it appears that the three-dimensional photonic band gap crystal structures prove more appropriate for uses such as antenna substrates. Basically, an aitenna radiates in a three-dimensional space. It follows therefrom that it is preferable to provide a structure ensuring a completely three-dimensional photonic band gap, that will be able to prevent any propagation of the electromagnetic radiation through it by covering all spatial directions, and that will consequently be effective as an omni-directional reflector,
A few three-dimensional configurations have been proposed in the prior art. Typical exemplary structures will be indicated below, and their main features will briefly be reminded, in connection with the manufacturing method.
U.S. Pat. 5,172,267 xe2x80x9cOptical reflector structure, method of fabrication, and communication methodxe2x80x9d, (filed by Eli YABLONOVITCH) describes a first type of photonic crystals with a three-dimensional band gap.
The appended FIG. 1A illustrates both the main features of the crystal structure la and the manufacturing process thereof. A mask 2, with a lattice of holes 2a regularly distributed in a triangular mesh configuration, is arranged on a crystal block 10a, simply. shaped as a rectangular parallelepipedal bar. Three successive micromachining operations are conducted, each of which provides straight holes or channels 100a in the crystal along a different determined orientation.
These micro-machining operations, for instance, can be performed by resorting to mechanical drilling, micro-lithography or ion beam technics, as suggested by the presence of three beams f1-f3 in FIG. 1a. More precisely, the three micro-machining operations can be performed along three different respective directions xcex941 to xcex943, each tilted by an angle xcex1 of predetermined identical amplitude with respect to a vertical axis xcex94V, orthogonal to the upper face of mask 2, and also consequently to both main faces of bar 10a. The angle xcex1 precisely equals 35.26 degrees. All three directions xcex941 to xcex943 are equally distributed in space; they consequently form angles of 120 degrees between one another. Those micro-machining operations result in a three-dimensional set of holes 100a providing empty channels in the material and crossing each another within the bar 10a. 
By appropriately selecting the physical characteristics (dielectric constant of the material forming the original crystal) as well as the geometric features (diameter of the holes in the mask, pitch distance between the adjacent holes), the structure with the desired three-dimensional photoninc band gap is obtained. For a more detailed description concerning the features of this structure according to the prior art, reference can be made to the above mentioned U.S. Pat. No. 5,172,267.
By appropriately selecting the physical characteristics (dielectric constant of the material forming the original crystal) as welle as the geometric features (diameter of the holes in the mask, pitch distance between adjacent holes), the structure with the desired three-dimensional photonic band gap is obtained. For a more detailed description concerning the features of this structure according to the prior art, reference can be made to the above mentioned patent U.S. Pat. No. 5,172,267.
However, it will be easily ascertained that the manufacturing method taught by the above prior art is quite complex, notably because three micro-machining, must be performed along three directions with different slopes with respect to the vertical axis xcex94V.
A second type of photonic crystal structures of the three-dimensional photonic band gap type is described in the U.S. Pat. No. 5,406,573 (Ekmel OZBAY and al.). The general configuration of the proposed structure, as described according to several alternatives, resembles a wood stack in which layers of logs, alternately oriented in two perpendicular directions, are piled successively on top of one another.
The appended FIG. 1B, which reproduces the main features of FIG. 2 in the above-mentioned prior art patent, illustrates one of the structure alternatives described in this document, said structure being designated as 1b in FIG. 1B. The 1b structure consists of a pile of rods 1b, arranged in layers on several levels, numbered N1 to N4 in FIG. 1B, and more generally N1 to Nn.
All rods are arranged parallel to an arbitrary reference plane with orthononnative axes X and Y, and rods located within a particular layer at a same level parallel to this plane are parallel to one another. Rods arranged in two if successive levels, for instance N1 and N2, are orthogonal to one another, i.e. they are alternately parallel to one then the other of the two axes x and Y of the universal two-dimension set x-Y, when going from one layer to the next one.
Furthermore, when considering two rows of rods 10b parallel to either one of the axes X or Y, with a being the lattice pitch representing the distance between two adjacent rods in the same layer, those rods belonging to the n-th rank level (for instance N4) will be shifted by half a pitch, i.e. a/2, from the rods belonging to the (nxe2x88x922)-th rank level (for instance N2). Consequently, rods having the same orientation are staggered from one level to the other, and this is true for both perpendicular orientations. In other words, the piling sequence is identically repeated every fourth layer.
Like in the previous case, the desired structure with a three-dimensional photonic band gap is obtained by appropriately selecting the physical properties (dielectric constant of the material forming the rods) and the geometric properties (rod diameter and pitch a which together determine a filling coefficient, representative of the ratio between the volume occupied by the material and the volume occupied by empty zones remaining between the rods). For a more detailed description of the features of this structure, reference can be made to U.S. Pat. No. 5,406,573 already mentioned.
Manufacturing this structure seems a priori less complex than for the preceeding one, inasmuch as the method consists in successively arranging rod layers on top of one another, similarly to a wood stack. Experience however shows that the thus obtained structure is brittle and has a low efficiency.
A third type of photonic crystal structure is described in the U.S. Pat. No. 5,600,483 (Shanhui FAN et al.). An example of such a structure is illustrated in the appended FIG. 1C, which corresponds in its main features to FIG. 1 of that patent.
The three-dimensional structure 1c is obtained from a basic rectangular parallelepipedic crystal block structure, having its main faces parallel to an arbitrary universal set XYZ. Block 1c consists of a plurality of two-dimension substructures 10c, made of stratums forming layers at different levels, shifted with respect to one another and parallel to the XY plane of the universal set XYZ. The different materials forming such stratums respectively have a first and a second dielectric constants:regions 10c and 101c, respectively. The final three-dimensional structure is obtained by providing channels 102c of a material with a third value for its dielectric constant. Channels 102c are arranged parallel to the Z-axis of the universal set XYZ.
Like in both previous cases, the desired structure with a photonic band gap is obtained by appropriately selecting the physical characteristics (dielectric constant values) and the geometric characteristics (channel diameter, distance between channel axes, relative stratums thicknesses, etc.) of the materials used. For a more detailed description of such structure features, reference can be made to the above U.S. Pat. No. 5,600,483.
This structure however is complex and requires a number of manufacturing steps.
It is the object of the present invention to remedy to the inconveniences of the prior art methods and devices, some of which were just stated, and more particularly to simplify the manufacturing of photonic crystals operating in all three spatial dimensions, to reduce their costs and to improve their performances, in particular when they are intended for applications in which they act as radiation reflectors and are used as antenna substrates in the production of telecommunication antennas.
The invention relates namely to a periodic dielectric structure of the three-dimensional photonic band gap type, having a simple configuration and combining the easy manufacture process of the one or two-dimension structures and the inherent efficiency of the complete three-dimensional structures.
For this purpose, the three dimensional structure is obtained from a structure with a very simple configuration, essentially of a unidirectional nature, consisting of a pile of stratums or layers of materials having different dielectric constant values, a priori two alternate dielectric constants. This basic structure is obtained by various manufacturing methods, well known per se, such as those using depositions or adherence techniques. A lattice of channels arranged according to a periodic two-dimension distribution is then provided through the structure having a one-dimension periodicity. The channels are bored perpendicular to the main faces of the basic structure layers, while respecting a particular pattern configuration with regular, for instance square, rectangular or triangular, meshes.
These arrangements of the invention allow obtaining a periodic dielectric structure with three-dimensional photonic band gap that simultaneously offers performances that; at least match those of the complete three-dimensional structures that have just been described and that do riot suffer from their complexity and do not involve any particular manufacturing difficulty.
For this purpose, it is appropriate for the refraction index of the internal medium in the channels to be representative of a third dielectric constant, different from both constants of the respective odd-numbered and even-numbered layers of the pile. The channels can be filled with an appropriate material, in particular with organic resins to be hardened in situ.
According to a preferred embodiment of the invention, particularly well adapted for applications in antennas operating in the atmosphere, the channels however are left open and empty and consequently will finally consist of air-filled holes. There follows that the dielectric constant is equal to unity, which allows benefiting from a strong dielectric contrast with the other structure materials.
According to its main aspect, the invention consequently provides a crystalline structure of the three-dimensional photonic band gap type
that comprises a pile of alternate series of layers of distinct dielectric materials respectively showing a first and a second determined dielectric constant values, wherein said layers in each series have a constant determined thickness between parallel main faces and said pile forms a substantially rectangular parallelepipedic block,
and further comprises a plurality of parallel channels provided inside said block and crossing same along a direction orthogonal to said main faces, wherein said channels are distributed according a two-dimension network frame or lattice and have a third determined dielectric constant value,
wherein said dielectric constant values and the relative geometric dimensions of said layers and said channels are selected so as to obtain said three-dimensional photonic band gap in a predetermined frequency range.
According to another aspect, the invention provides a manufacturing method for a crystalline structure of three-dimensional photonic band gap type, comprising the following steps constructing a pile made of alternate layers of distinct materials, wherein each of said materials layers substantially forms a rectangular parallelepiped of a determined thickness and said piles constitute said substantially rectangular parallelepipedal block, and said materials show said first and said second determined electric constant, and providing a plurality of channels within said block, along a direction orthogonal to the main faces of said alternate layers.
According to this invention, the desired three-dimensional photonic band gap is obtained in a predetermined frequency range by appropriately selecting the nature of the materials used and the values of the corresponding dielectric constants, the pile layer thickness, the mesh type, the channel lattice pitch, as well as the channel section and, generally, all the physical properties of the various structure components and the geometric dimensions that characterize the layers and channels.